Feral cats of Kangaroo Island are part of its ecosystem — Dr Bittar, 2015.04.15

There are plans being discussed for the extermination of all feral cats on the island. Here’s a short, out-of-the-box contribution to the debate.

Complexity being complexity, as a rule any drastic action has more chances of bearing negative consequences than positive ones. Ecology is indifferent to the likes and dislikes of its actors, including humans, and nature fills any void quickly.

In the absence of rabbits, by far the main component of the diet of the island feral cats is mice and rats. So, if all wild cats are exterminated on Kangaroo Island, what will, straightaway, replace them ecologically in the present context?

Predatory rats. They are numerous, and reproduce very quickly. Anyone who’s tried to run an orchard here has found it mission impossible because of the rats.

Rats are discrete nocturnals; barn owls are too rare to keep them in check, and the only other nocturnal bird of prey, the boobook Ninox, is no match to them.

Rats are as good as cats at climbing up trees, but in addition they can go into places unreachable to cats: narrow burrows. They also eagerly look for eggs, which cats don’t. They are fierce fighters and predators. Without their numbers being controlled by wild cats, their impact on birds and marsupials would be higher by an order of magnitude.

Black tiger-snakes could possibly increase in numbers thanks to the higher availability of mice and rats, but they would probably not indent the increasing rat population — too much of a reproduction-rate differential, and obviously the rats would predate on the snakes’ eggs and youngest…

These are a few simple points, though with potential enormous impact, which the decision-makers should ponder carefully.

But there’s another side to the story, more subtle, yet even more overwhelming from an ecological point of view.

Being obligatory carnivores, wild cats as predators on the island are much less detrimental to the local birds and marsupials than would any pervasive and unchecked omnivorous predator such as the rat. In a smaller ecosystem, omnivorous predators have much more potential for irreversible imbalance than the obligatory carnivorous ones. The reason being that they are much less subject to the population dynamics of the Lotka-Volterra prey-predator model (two oscillating population-curves advancing out of sync but at the same rythm): the omnivorous predators can easily switch to a vegetarian mode, thus avoiding their own population crash which would otherwise have followed their prey’s population crash. In such a situation, the prey population has scant chance to rebuild its numbers, being confronted to still numerous predators while itself having become rarified.

That’s why, ecologically, omnivorous rats and feral pigs are much more hazardous to a smaller ecosystem than any obligatory carnivores such as feral cats.

Dr Gabriel Bittar, Kangaroo Island

Here are a few references for those who wish to check out on some of the latest research.

The Invasive IdeologyBiologists and conservationists are too eager to demonize non-native species“, Chew and Carroll 2011, The Scientist online 2011.09.07

The island syndrome and population dynamics of introduced rats“, Russell at al. 2011, Oecologia 167:3 pp 667-676

Near-complete extinction of native small mammal fauna 25 Years after forest fragmentation“, Gibson et al. 2013, Science 341:6153 pp. 1508-1510

A continental-scale analysis of feral cat diet in Australia“, Doherty et al. 2015, Journal of Biogeography, Doherty et al. 2015, online 2015.02.02

Without feral cats, Kangaroo Island would be a Rat Island — Dr G. Bittar 2013.09.30

Macquarie Island vegetation devastated because of extermination of its cats, Bergstrom et al. 2009

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